1. Field of the Invention
The present invention relates to an image forming apparatus employing the electrophotographic method and, more particularly, the invention relates to an image forming apparatus for obtaining an image-formed article by transferring a toner image formed on a first image bearing member onto an intermediate transfer belt and again transferring the image onto a transfer medium as being a second image bearing member.
2 . Related Background Art
Image forming apparatus incorporating the intermediate transfer belt is well known and is effectively applicable to color image forming apparatus and multi-color image forming apparatus for successively transferring and stacking a plurality of component color images based on color image information or multi-color image information to synthetically reproduce a color image or a multi-color image, thereby obtaining an image-formed article, or to image forming apparatus provided with such color image forming function or multi-color image forming function.
An example of the image forming apparatus incorporating the conventional intermediate transfer belt is schematically shown in FIG. 7. This image forming apparatus is constructed as a color image forming apparatus (a copier or a laser beam printer) of the electrophotographic process having the intermediate transfer belt 20. This intermediate transfer belt 20 is made of an elastic member of medium resistance.
The image forming apparatus is provided with an electrophotographic, photosensitive member of a drum shape (hereinafter referred to as "a photosensitive drum") 1 as a first image bearing member and this photosensitive drum 1 is driven to rotate at a predetermined peripheral velocity (process speed) in the direction of the arrow.
The photosensitive drum 1 is uniformly charged in predetermined polarity and potential by primary charger 2 during the rotating process thereof, and is then exposed to image exposure light 3 by image exposure means not illustrated. This forms an electrostatic, latent image corresponding to a first color component image (for example, a yellow color component image) of an aimed color image.
Then the electrostatic, latent image is developed at a developing position by first developing unit (yellow developing unit) 41 to be visualized as a yellow toner image. At this time second to fourth developing units, i.e. magenta developing unit 42, cyan developing unit 43, and black developing unit 44, are kept unoperated so as not to act on the photosensitive drum 1, so that the yellow toner image is not subject to the action of the second to fourth developing units 42 to 44. The first to fourth developing units 41 to 44 are mounted so as to be rotatable on support member 40 and are successively moved to the developing position opposed to the photosensitive drum 1.
The intermediate transfer belt 20 is looped over two rollers 61 and roller 64 and is then driven to rotate at the same peripheral velocity and in the same direction of movement at the opposed portion in contact with the photosensitive drum 1 as the photosensitive drum 1 does. Primary transfer roller 25 is located at a position inside the intermediate transfer belt 20 in the contact portion with the photosensitive drum 1, whereby a primary charging bias from bias supply 29 is applied through the primary transfer roller 25 to the intermediate transfer belt 20. The primary transfer bias is of the opposite polarity to toner and the applied voltage is, for example, in the range of +100 V to +2 kV.
The yellow toner image formed on the photosensitive drum 1 is successively transferred to the outside peripheral surface of intermediate transfer belt 20 by a primary transfer electric field created by the primary transfer bias applied from the primary transfer roller 25 to the intermediate transfer belt 20, during passage through the contact nip portion between the photosensitive drum 1 and the intermediate transfer belt 20 (primary transfer).
The photosensitive drum 1, after completion of transfer of the yellow toner image of the first color onto the intermediate transfer belt 20, is cleaned by cleaning unit 13 to remove the residual toner remaining on the surface after the primary transfer and is thereafter subjected to image forming processes to follow the primary charging. Then a magenta toner image of the second color, a cyan toner image of the third color, and a black toner image of the fourth color are formed in the same manner to be transferred as successively stacked on the intermediate transfer belt 20, thereby obtaining a synthetic color image corresponding to the aimed color image.
The roller 64 supporting the intermediate transfer belt 20 is a secondary transfer opposite roller and secondary transfer roller 63 is arranged as retractable at a position on the outside periphery of the intermediate transfer belt 20 where the roller 64 is located. A secondary charging bias is applied from bias supply 28 to the secondary transfer roller 63. The secondary transfer roller 63 can be spaced away from the intermediate transfer belt 20 during the primary transfer steps of the toner images of the first color to the third color.
The secondary transfer bias is applied from the bias supply 28 to the secondary transfer roller 63 at the timing when the toner images of the four colors transferred in superimposed fashion on the intermediate transfer belt 20 reach the vicinity of the secondary transfer portion by rotation of the intermediate transfer belt 20. At the same time, the secondary transfer roller 63 is brought into contact with the intermediate transfer belt 20. Further, a transfer medium (paper or resin sheet) P as a second image bearing member is fed at predetermined timing to the contact portion by sheet feed roller 11 to be supplied thereto through guide 10.
The toner images of the four colors on the intermediate transfer belt 20 are successively transferred together onto the surface of transfer medium P by a secondary transfer electric field created by the secondary transfer bias applied from the secondary transfer roller 63 to the intermediate transfer belt 20, during passage through the contact nip portion between the intermediate transfer belt 20 and the secondary transfer roller 63 (secondary transfer). The transfer medium P after the secondary transfer of the four-color toner images is then guided to fixing unit 15, in which it is heated and pressed to fuse and mix the four-color toner components and to fix them on the transfer medium P, thereby forming a full-color printed image.
The toner of secondary transfer residue remaining on the surface of the intermediate transfer belt 20 is charged in the opposite polarity to the photosensitive drum 1 by belt cleaner 8. The belt cleaner 8 is comprised of a roller arranged as retractable on the outside periphery of the intermediate transfer belt 20. The secondary transfer residue toner is charged to the predetermined polarity by applying a cleaning bias of the predetermined polarity to the belt cleaner 8 by bias supply 26 while keeping the belt cleaner 8 in contact with the surface of the intermediate transfer belt 20 and using earthed conductive roller 7 as an opposed pole located inside the intermediate transfer belt 20. In this example the photosensitive drum 1 is charged in the negative polarity, and the secondary transfer residual toner is thus charged in the positive polarity. The belt cleaner 8 can be spaced away from the intermediate transfer belt 20 during the primary transfer steps of the toner images of the first color to the third color.
The secondary transfer residual toner charged in the opposite polarity on the intermediate transfer belt 20 is electrostatically attracted to the photosensitive drum 1 in the contact portion of the intermediate transfer belt 20 therewith and in the vicinity thereof, thereby being removed from the intermediate transfer belt 20.
The above color image forming apparatus incorporating the intermediate transfer belt can transfer the toner image from the intermediate transfer belt to the transfer medium without a need for any control on the transfer medium (for example, gripping the transfer medium by a gripper of transfer drum, attaching the transfer medium onto the surface of transfer drum, giving the transfer medium curvature along the surface of transfer drum, etc.), when compared with the color image forming apparatus arranged to stick or attach the transfer medium onto the transfer drum for carrying it and to transfer a toner image of each color from the photosensitive drum onto the transfer medium to obtain a color image, for example, as described in Japanese Laid-open Patent Application No. 63-301960. Therefore, the above apparatus has such an advantage that the color image can be obtained by transferring of the toner image, regardless of whether the width is wide or narrow or whether the length is long or short, ranging from thin paper of about 40 g/m.sup.2 to thick paper of 200 g/m.sup.2, such as an envelope, a post card, or a label sheet.
Because of this advantage, the color image forming apparatus incorporating the intermediate transfer belt is already in operation as a color copier, a color printer, or the like.
The conventional image forming apparatus incorporating the intermediate transfer belt, however, had the following problems, though having the above advantage; (1) void image often appears as shown in FIG. 8; (2) because of expansion and contraction of the intermediate transfer belt 20 at the portion of roller 61 as shown in FIG. 9 (FIG. 9 schematically shows the expansion and contraction of intermediate transfer belt 20 by illustrating stripes at equal intervals in the cross section of intermediate transfer belt 20) and mechanical vibration transferred to the intermediate transfer belt 20, the primary transfer toner is readily scattered from the intermediate transfer belt 20 during rotation thereof (scattering on the belt), which degrades the quality of image.